NEED HELP FAST>>>PLEASE!!!
I have had the same exact problem recently. It happened after I installed a 160 therm.. I didn't drain the coolant completely while replacing the thermostat, so when I hooked the hose back up, that intoduced an air pocket into the system. So, when I started my car up, the gauge would rise until it hit normal temp. then it would fall to zero. I called everyone under the sun about the problem. I finally contacted to Mark Schikel from SLP Engineering. He told me that I had air in the system and that I needed to apply a small amount of vacuum to my throttle body coolant line. So I did and it spit and sputtered for a while then ran smooth with coolant. Fixed my problem completetly. I never threw a code though, that was weird? Anyhow, good luck on your problem, it doesn't sound like air would be the problem, you'll find it.

